Sequence stratigraphic reappraisal of the nature of transition between two lithostratigraphic units (formations) - A study from the Permian Lower Gondwana succession, West Bokaro Basin, Eastern India

Understanding depositional history through evaluation of lithostratigraphic units, e.g., a Formation, is often incomplete, as identification of a lithostratigraphic unit solely includes rocks with identical lithological attributes within the basin-fill succession. Re-assessment of sedimentary successions in a sequence stratigraphic framework, irrespective of established lithostratigraphy, portrays more complete depositional history by correlation of genetically-linked deposits across the basin. In this backdrop, the present paper re-examines the nature of transition between two lithostratigraphic units from the Permian Lower Gondwana succession, viz., the coalbearing Barakar Formation and the overlying non-coaliferous Barren Measures Formation, from the West Bokaro Basin in peninsular India. The transition is conformable, but conjectural in terms of its proper demarcation on outcrops. In-depth facies analysis of the uppermost part of the Barakar Formation and the lowermost part of the Barren Measures Formation demarcates continuous deposition under a fluvio-marine interactive estuarine condition with significant tide and wave influences. A high resolution sequence stratigraphic model signifies part of a 3rd order transgression (a transgressive systems tract, TST), implied by overall continuous fining-up succession with more marine influences towards the top. Within the 3rd order TST, demarcation of 4th order systems tracts, viz., a lowstand systems tract (LST), multiple transgressive systems tracts (TSTs) and a highstand systems tract (HST), defined by hemicycles with distinct stratal stacking patterns and bounding surfaces, signify fluctuations in the net accommodation due to short-term changes in base level and sediment supply. Such fluctuations point to the prevalent autogenic and allogenic forcings in the Permian Gondwana basins in peninsular India.